The alkylation of aromatic hydrocarbons such as benzene with olefins having two, three, or four carbon atoms (hereinafter referred to as “light” olefins), such as ethylene and propylene, is a commercially important process. The production of ethylbenzene is used to provide a feedstock for styrene production, while the alkylation of benzene with propylene produces isopropylbenzene (cumene). Cumene is an important feedstock to make phenol as well as a good gasoline blending component. Numerous other uses exist for such alkylated aromatic hydrocarbons. In these alkylation processes, new catalysts are continuously needed that have a high overall conversion of the feedstock and have a good selectivity of alkylated benzenes.
Zeolite catalysts (hereinafter referred to collectively as “zeolites”) have been found to be particularly well-suited for use in the alkylation of aromatic hydrocarbons. Zeolites are crystalline aluminosilicate compositions that are microporous and that are formed from corner sharing AlO2 and SiO2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared, are used in various industrial processes. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al, and structure-directing agents such as alkali metals, alkaline earth metals, amines, and/or organoammonium cations. The structure-directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolite catalyst compositions also typically include a filler or binder material mixed with the aluminosilicate material, which helps to form the zeolite catalyst into a desired shape. Zeolites are characterized by having pore openings on the external surface of the catalyst, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase, which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure.
While zeolites have been shown to be useful for the alkylation of a variety of aromatic hydrocarbons with a variety of light olefins, testing has shown that alkylation reactions using zeolite catalysts are highly diffusion limited. The reaction rate is so fast and the diffusion of olefin into and throughout the catalyst to reach all the zeolitic active sites is the rate limiting step. Diffusion within the catalyst depends, in part, on the overall porosity and pore structure of the catalyst and in part on the dispersion of zeolite throughout the matrix. In order to disperse the zeolite throughout the matrix mechanic force through a process such as mulling is applied. However, the compaction invariably reduces the catalyst porosity, increasing the diffusion resistance and thus lowering the catalyst activity.
Accordingly, it is desirable to provide methods for producing zeolite catalysts that have better diffusion characteristics and thus improving the utility of zeolite throughout the entire catalyst. The effect of diffusion resistance on active site utilization and thus catalyst activity is especially evident in the aromatic alkylation with olefin including propylene and ethylene. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.